Such a motor-compressor unit is known from U.S. Pat. No. 4,810,915. FIG. 1 of said Patent shows the coupling 11 between the rotor arm 12 and the pistons 10. FIG. 3B of said Patent shows the coupling in another sectional view. This coupling is a slider mechanism, the operation of which will be explained below with reference to FIGS. 1 to 3.